1) Field of the Invention
The present invention relates to an optical switch, and particularly to an optical switch suitable for performing path switching for each wavelength component.
2) Description of the Related Art
At present, it is well known that networks centered on wavelength division multiplexing (WDM) have been rapidly made optical in order to accommodate remarkably increasing Internet traffics. Though the present WDM is mainly in the form of point-to-point network, there is considered that the network would be developed to a ring-shaped network or mesh-shaped network in the near future, each node constituting a network could perform processings such as add/drop of arbitrary wavelength and overall optical cross connect (OXC) without conversion into electricity, and setting/releasing of dynamic path based on wavelength information would be performed. The development of photonic network making the most of such optical technique is described in detail in, for example, the journal of IEICE (Institute of Electronics, Information and Communication Engineering) pp. 94-103, 2002-2.
In accordance with the development of the photonic network technique as described above, the development of an optical switch (wavelength selective switch) for performing path switching for each wavelength component is in progress. The wavelength selective switch has a function of classifying an input wavelength to an arbitrary output port, and is arranged at a node in the ring-shaped network or mesh-shaped network as described above to assist realization of a network having a function of setting/releasing a dynamic path based on wavelength information.
FIG. 8 is a diagram showing a conventional optical switch having a wavelength selective switch function. A similar optical switch is disclosed also in the following Patent Reference 1. An optical switch 100 shown in FIG. 8 comprises a collimator 101, a spectral unit 102, a lens 103 and a micromirror array 104. An input optical fiber 105-1 and output optical fibers 105-2 to 105-4 are vertically arranged in the collimator 101.
The collimator 101 outputs a light from the input optical fiber 105-1 as a parallel light to the spectral unit 102, and guides output lights as reflected lights from the spectral unit 102 to the output optical fibers 105-2 to 105-4. The spectral unit 102 is constituted of, for example, a diffraction grating, and is directed for spectrally diffracting a light constituting a wavelength component of the input light from the collimator 101 horizontally in the figure.
FIG. 9 is a diagram showing a sectional structure of a diffraction grating 102A capable of being used as the spectral unit 102. As shown in FIG. 9, the diffraction grating 102A is constituted so that a large number of parallel grooves 102-2 are periodically carved on a glass substrate 102-1. A light LI having a plurality of wavelength components incident at a constant angle α is given an angle β different for each wavelength in an output light LO using a diffraction phenomenon of light. Thus, the wavelength components can be separated.
The lens 103 is directed for focusing each wavelength component light spectrally diffracted in the spectral unit 102, and the micromirror array 104 comprises a plurality of micromirrors 104a which are reflectably arranged at light focusing points of the lens 103 for each wavelength light, respectively. This micromirror array 104 is constituted of MEMS (Micro Electro Mechanical System) mirror array as shown in FIG. 10 and can independently perform switching control of a reflecting angle of the micromirror 104a for reflecting each wavelength component light.
In the optical switch 100 constituted in this manner, a light having several wavelength components input from the input optical fiber 105-1 is spectrally diffracted in the spectral unit 102 and the spectrally-diffracted wavelength component lights are reflected on the corresponding micromirrors 104a. That is, one wavelength of spectrally-diffracted wavelength component light is reflected on the corresponding micromirror 104a. 
At this time, switching control is performed on an inclination angle in each micromirror 104a so that the output optical fiber 105-2 to 105-4 as an output destination can be selectively switched for each wavelength component.
For example, when the output destination is the output optical fiber 105-4, as shown in FIG. 11(a), the inclination angle of the micromirror 104a is set such that a wavelength component light reflected on the micromirror 104a is guided to the output optical fiber 105-4 via a path through the lens 103, the spectral unit 102 and the collimator 101. Similarly, when the output destination is the output optical fiber 105-3, the inclination angle of the micromirror 104a is set as shown in FIG. 11(b).
Thereby, the output optical fiber 105-2 to 105-4 as the output destination of each wavelength component is set according to the inclination angle of the corresponding micromirror 104a for the wavelength component constituting the input light.
In order to achieve small-sizing of the optical switch 100 having the function described in FIG. 8, it is important to enhance the amount of linear dispersion (capability of decomposing a wavelength or deflecting angle per unit wavelength) of the spectral unit (refer to numeral 102 in FIG. 8) described later. FIG. 12 is a diagram for explaining a relationship between the setting of the amount of linear dispersion and a focal distance of the lens 103.
As shown in FIG. 12, it is assumed that a light decomposed by the amount of linear dispersion defined by a derivative dθ/dλ of an output angle θ relative to a wavelength λ is made to be a parallel light in the lens 103 arranged at an interval of the focal distance “f” from the spectral unit 102 and to be incident into the micromirror array 104. A WDM light where several wavelength components having a wavelength interval Δλ are multiplexed is spectrally diffracted in the spectral unit 102 to be parallel lights in the lens 103, respectively.
At this time, in order to reflect each wavelength component light as a parallel light at the minimum loss on the micromirror 104a, it is desirable to match the separation of each wavelength component light with a mirror pitch “p” of each micromirror 104a constituting the micromirror array 104. In order to make the separation of the separated parallel light equal to the mirror pitch “p”, it is necessary to meet formula (1).p=Δλ·dθ/dλ·f  (1)
Here, when the mirror pitch and the wavelength interval Δλ are set, the focal distance “f” is made smaller to enable an arrangement of an optical system to be compact so that small-sizing of the optical switch can be developed. However, in order to make this focal distance “f” small, when the mirror pitch and the wavelength interval Δλ are set as described above, it is necessary to make the amount of linear dispersion dθ/dλ larger.
In order to make the amount of linear dispersion larger, it is considered to increase the number of grooves per unit length of the diffraction grating or to user several diffraction gratings.
There will be initially considered the case of increasing the number of grooves per unit length of the diffraction grating. FIG. 13 is a diagram for explaining a relationship between an incident angle into the diffraction grating and the amount of linear dispersion in changing “Nm”. Here, “N” indicates the number of grooves and “m” indicates an order of diffraction. When the order of diffraction “m” is assumed to be fixed at “1”, the number of grooves “N” is increased so that the amount of linear dispersion can be made larger. But in the typical diffraction grating 102A as shown in FIG. 9, as the number of grooves is increased, reduction in diffraction efficiency, increase in polarization dependent loss (PDL) and the like occur.
Though Patent Reference 2 listed below describes a diffraction grating capable of restricting the reduction in diffraction efficiency even when the number of grooves is increased, since the diffraction grating has a specific structure, manufacturing cost would be higher at present.
There will be next considered the case of using several diffraction gratings. FIG. 14 illustrates a structure of using two diffraction gratings 112-1 and 112-2 to spectrally diffract. In FIG. 14, the two diffraction gratings 112-1 and 112-2 are cascade-arranged on an optical path so that an optical signal spectrally diffracted in the first diffraction grating 112-1 is further transmitted through (or reflected on) the other diffraction grating 112-2, thereby doubling the dispersion capability.
For example, when a wavelength-multiplexed light having wavelength components λ1 to λ3 is incident at an incident angle α in the first diffraction grating 112-1, the wavelength component lights λ1 to λ3 are output at angles α−Δθ, α and α+Δθ different from each other, and when the wavelength component lights λ1 to λ3 are incident into the second diffraction grating 112-2 at the incident angles α−Δθ, α and α+Δθ, the lights are further diffracted and output, respectively. Patent reference 3 also describes a structure where two diffraction gratings are cascade-arranged on an optical path.
The diffraction grating, which has the relatively small amount of linear dispersion, can achieve both relatively high diffraction efficiency and relatively small PDL with relative ease, and can be constituted at low cost. Therefore, the structure having the two diffraction gratings which are cascade-arranged (double monochrome structure) shown in FIG. 9 may be good for constituting the spectral unit meeting the relatively high amount of linear dispersion, the relatively high diffraction efficiency, the relatively small PDL and the relatively low cost without increasing the number of grooves of each diffraction grating.
Additionally, techniques related to the present invention include Patent Reference 4 and Patent Reference 5 below.
(Patent Reference 1) U.S. Pat. No. 6,549,699
(Patent Reference 2) U.S. Pat. No. 6,750,995
(Patent Reference 3) U.S. Patent Application Publication No. 2002/0154855
(Patent Reference 4) U.S. Pat. No. 6,583,934
(Patent Reference 5) U.S. Patent Application Publication No. 2002/0109076
However, in the conventional optical switch 100 as described above, when the spectral unit 102 is constituted so that several diffraction gratings are cascade-arranged, there is a problem that when spectrally-diffracted wavelength components are focused in the lens 103, an angular deviation occurs for the optical axis direction, that is, even when the optical axis of each wavelength component transmits through the lens 103, the wavelength components are difficult to be parallel.
The lights (wavelength component lights) transmitting through the lens 103 are difficult to be parallel because of the following reason. That is, as shown in FIG. 14, since the wavelength component lights λ1 to λ3 are output in the first diffraction grating 112-1 at the angles α−Δθ, α and α+Δθ different from each other, respectively, the lengths of the optical paths op1 to op3 up to being incident into the second diffraction grating 112-2 are different from each other and the incident angles α−Δθ, α and α+Δθ into the diffraction grating 112-2 are also different from each other.
In other words, assuming that the light beam of each wavelength component output from the second diffraction grating 112-2 is swept, the light beams do not cross at one point. That is, the imaginary focal position is different, so that even when the light beams are transmitted through the single lens 103 having the focal distance “f”, the light beams are difficult to be parallel.
Therefore, when the spectral unit 102 of the optical switch 100 employs the double monochrome structure as shown in FIG. 14, each wavelength component is not perpendicularly incident into the micromirror array 104, thereby deteriorating an efficiency that the reflected lights in the micromirror array 104 are coupled with the output optical fibers 105-2 to 105-4.
Also in the techniques described in Patent references 1 to 5, there is not described a technique for improving a reduction in coupling with an output optical path in the case where several diffraction gratings are cascade-arranged.